1. Field of the Invention
This invention relates to an apparatus for recording or reproducing a video signal by radially shifting a head on a disc-shaped recording medium (hereinafter referred to as the disc) to form thereby recording tracks either in a concentric circular shape or in a helical shape and more particularly to a method for driving the head carriage of an apparatus of such kind.
2. Description of the Related Art
Recording density is trending upward as a result of the recent advancement of reduction in size of a magnetic disc recording and/or reproducing apparatus. The width and pitch of the recording tracks to be formed on the disc are being set only at some scores of .mu.m. In positioning the head of the apparatus on the disc, therefore, a high degree of precision is required to limit an allowable error of the position of the head relative to the disc to only several .mu.m.
The above stated requirement is particularly severe in the case of the so-called electronic still camera. The standardized diameter of a magnetic disc to be handled by a device incorporated in the camera of that kind is only 47 mm. Since the camera must be compact somewhat like the conventional camera, both the allowable size and the power consumption of the device which includes means for positioning the head are severely limited.
The conventional disc handling devices of this kind have been provided with varied arrangements using a lead screw, a cam, etc. for stepwise feeding of the disc.
Most of these devices are arranged to linearly drive a head carriage by driving the lead screw, the cam or the like by means of a drive source such as a stepper motor via a reduction device which has a gear train or the like arranged in relation to the drive source. In this instance a head positioning error results from an engagement play existing in the gear train. To solve this problem, practice has been to to apply to the gears a unidirectional rotation urging force to absorb the play among them.
FIG. 1 of the accompanying drawings shows an example of the conventional methods. The illustration includes a portion of a head carriage 1 which is loaded with a head and is linearly guided in the direction of arrow A. A cam follower pin 1a is erected on the carriage 1 and is in a position to abut on the periphery of a feed cam 2. The head carriage 1 is urged to move to the left as viewed on the drawing by a spring which is not shown. Therefore, the cam follower pin 1a is lightly pushed into resilient contact with the periphery of the feed cam 2.
The feed cam 2 is pivotally carried by a shaft 4 to be rotatable on the shaft 4 and is thus arranged in one unified body with a gear 3. A rotating force of a stepper motor which is not shown is transmitted via a gear 7, etc. to cause the feed cam 2 to turn round at a given angle relative to the rotation angle of the stepper motor. The head carriage 1 is arranged to be linearly shifted in the direction of arrow A accordingly as the feed cam 2 rotates to change the radial position thereof. An engagement-play-absorbing coiled spring 5 is mounted on the periphery of a boss part 3a which is formed in one body with the gear 3. One end of the engagement play absorbing spring 5 is attached to a spring peg 3b erected on the gear 3. The other end of the spring 5 is secured to a chassis which is not shown. The gear 3 which is unified with the feed cam 2 is urged to turn clockwise by the tension of the coiled spring 5 exerted in the direction of arrow B in such a way as to absorb the engagement play in question.
In the case of the above stated arrangement, the feed cam 2 makes about one turn while the head is shifted from its outermost position to an innermost position on the disc. Accordingly, the tension of the spring 5 changes from a maximum value to a minimum value thereof upon completion of one turn of the cam.
With the device thus arranged to change the one-way pulling force for the purpose of absorbing the engagement play, however, the head positioning precision would still be affected by fluctuations in the head stopping position relative to each phase of the stepper motor in cases that the device is required to have an absolute position accuracy within an error range of only several .mu.m.
In another known method for preventing an engagement play between gears, two gears of the same diameter are arranged one on top of another in the direction of their rotation axis; a spring is arranged between the gears to urge them to turn in opposite directions; and then other gears which transmit a driving force are mounted in a state of engaging the former gears on the same plane. In other words, in accordance with that method, the engagement with other gears prevents the two gears from turning round in the opposite directions. Any play between the gears thus can be absorbed while the driving or rotating force is transmitted with the gears rotating together. In accordance with that method, however, it is always only the play between two gears that is removable. In the event of a driving force transmission system consisting of three or more than three gears, the same arrangement must be made for each pair of engaging gears. In that event, the play between one pair is not the same as the play between another pair of gears and, therefore, it is impossible to remove the engagement plays of the whole driving force transmission system.
Besides, the method necessitates the forming of each gear with two vertically overlapped gears and to provide urging springs between them. The method is, therefore, too complex in structural arrangement and thus also has a shortcoming in terms of workability.